This invention relates to papermaking. More particularly, this invention is concerned with the manufacture of grades of paper that are suitable for use in paper toweling, napkins, facial tissue, and bathroom tissue by methods that include creping utilizing novel adhesives used as creping process aids.
In the manufacture of tissue and towel products, a common step is the creping of the product. This creping is done to provide desired aesthetic and performance properties to the product. Many of the aesthetic properties of tissue and towel products rely more upon the perceptions of the consumer than on properties that can be measured quantitatively. Such things as softness, and perceived bulk are not easily quantified, but have significant impacts on consumer acceptance. Since many of the properties of tissue and towel products are controlled or are at least influenced by the creping process, it is of interest to develop methods for controlling the creping process. Although the creping process is not well understood, it is known that changes in the process can result in significant changes in the product properties. A need exists to provide a method for influencing the creping process by allowing the control of the adhesion of the tissue or towel substrate to the surface from which it is creped, most usually large cylindrical dryers known in the industry as Yankee dryers.
Paper is generally manufactured by suspending cellulosic fibers of appropriate length in an aqueous medium and then removing most of the water to form a web. The paper derives some of its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most by far of the paper""s strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this interfiber bonding, while necessary to the utility of the product, results in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drum thermal drying means with an adhesive/release agent combination and then scraping the web off of the Yankee by means of a creping blade. Creping, by breaking a significant number of interfiber bonds, increases the perceived softness of the resulting bathroom tissue product.
In the past, common classes of thermosetting adhesive resins which have been used as Yankee dryer adhesives have been represented by poly (aminoamide)-epichlorohydrin polymers (hereinafter referred to as PAE resins), such as those polymers sold under the tradenames Kymene, Rezosol, Cascamid, and Amrezs. Each of these materials represent products sold respectively by the Hercules Chemical Company, the Houghton Company, the Borden Company, and Georgia-Pacific. Although these materials are now in commercial use, our novel adhesive formulations are environmentally friendly and have lower in-use cost.
This invention provides adhesion which is equal or better than the adhesion characteristics available through the use of PAE resins but having none of the attendant environmental problems associated with the halogen moiety. The halogen free, particularly chloride free, Yankee dryer adhesives of this invention prevent or inhibit chloride or halogen induced corrosion of the Yankee drum surface and, also, are friendly to the environment and have a lower in use cost.
Obtaining and maintaining adhesion of tissue and towel products to Yankee dryers is an important factor in determining crepe quality. Inadequate adhesion results in poor or non-existing creping, whereas excessive adhesion may result in poor sheet quality and operational difficulties. Traditionally, creping adhesives alone or in combination with release agents have been applied to the surface of the dryer in order to provide the appropriate adhesion to produce the desired crepe. Various types of creping adhesives have been used to adhere fibrous webs to dryer surfaces such as Yankee dryers. Some examples of prior art creping adhesives are disclosed in U.S. Pat. Nos. 4,886,579; 4,528,316 and 4,501,640.
Prior Art of interest includes Smigo U.S. Pat. No. 5,232,553, Miyosawa U.S. Pat. No. 4,016,126, and Hollenberg, et al., U.S. Pat. No. 5,246,544. None of these relate to the creping adhesives of this invention. The Smigo patent discloses certain combinations of polyvinylamides suitable for reducing fines in the paper making process. Smigo""s patent is specifically directed to retaining fines from recycle of waste papers. The Miyosawa patent is directed to hardenable coating compositions, particularly films. The coatings consist of silica-polyvinyl-alcohol complexes and are unrelated to the creping adhesives disclosed herein.
The Hollenberg et al., U.S. Pat. No. 5,246,544 discloses a preparation of an adhesive from polymers not containing amine moieties wherein the adhesive is prepared prior to its application on a dryer. The amine moiety containing copolymers of this invention are neither disclosed nor suggested by Hollenberg. The Hollenberg reference is not directed to the adhesives disclosed herein since the adhesives of this invention are prepared on the Yankee surface. The reactivity of the components of the adhesives of this invention are such that if they were mixed together prior to spraying on the Yankee surface a polymerization of the components would take place which would be useless as creping adhesives. The Hollenberg adhesives cannot be prepared on the Yankee surface since they do not contain amine moieties which can interchange with the carbon containing moiety of the zirconium crosslinking agent of this invention.
U.S. Pat. No. 5,246,544 describes a creping adhesive that provides the ability to control coating mechanical properties and adhesion, and which can be more easily removed from dryer surfaces. The adhesive system described in said patent provides high adhesion of a fibrous web to a dryer surface with low xe2x80x9cfrictionxe2x80x9d. Having low friction means that the fibrous web can easily be removed from the dryer surface. Other references of interest include U.S. Pat. Nos. 5,232,553 and 4,684,439. All the prior art patents are of interest but do not disclose polymers having at least one primary or secondary amine group in the backbone such as chitosan, polyvinylamine, polyvinyl alcohol-vinyl amine, polyaminoamide and etc., in combination with the zirconium crosslinking compounds having a valence of plus four such as ammonium zirconium carbonate, zirconium acetylacetonate, zirconium acetate, zirconium carbonate, zirconium sulfate, zirconium phosphate, potassium zirconium carbonate, zirconium sodium phosphate and sodium zirconium tartarate. In our process, the creping adhesive is formed on the Yankee surface wherein the carbon containing moiety of the zirconium crosslinking agent is exchanged with the amine moiety of the copolymer. The vinylamide copolymer also crosslinks with the cellulose moiety of the absorbent paper. These patents also do not relate to creping adhesives or the creping of tissue and towel from a Yankee dryer. In our novel process the crosslinking agents are charged to the Yankee surface at the same time as the adhesive polymer wherein the adhesive of this invention is formed on the Yankee surface.
The present invention provides creping adhesives which are friendly to the environment giving off no chlorine compound pollutants, can be applied directly to the Yankee from aqueous solution and are substantially less costly than the presently available creping adhesives. The present invention provides an improved creping adhesive which provides the ability to readily control glass transition (Tg) and adhesion and which can be more easily removed from dryer surfaces.
An advantageous feature of the present invention is that the adhesion properties of specific types of polymers or copolymers (hereinafter referred to as base polymers) can be systematically changed by varying the amount of crosslinking that may occur when the base polymer is dried onto the surface of a Yankee dryer with the zirconium crosslinking agents. Because crosslink density influences the mechanical properties (i.e., modulus, brittleness, Tg), this permits the adjustment of adhesion/release of the fibrous substrate onto the surface of the dryer. Base polymers having at least one primary or secondary amine groups in the backbone such as chitosan, polyvinylamine, polyvinyl alcohol-vinyl amine, polyaminoamide and etc., crosslinked with zirconium compounds having a valence of plus four produces an adhesive friendly to the environment and which is much less costly than the PAE resin available on the market as discussed in the background section. The invention also relates to a process for applying such base polymers without pre-crosslinking to achieve adhesion control on the paper machine through spray application. This invention also relates to creped fibrous webs, creped tissue and creped towel and a process for the manufacturing of these paper products utilizing the novel adhesives of this invention.